DESCRIPTION: (Applicant's Abstract) Alkylating agents remain the backbone of chemotherapy for many malignant tumors. Well-defined mechanisms of alkylating agent resistance prevent optimal therapeutic response. The applicant proposes to investigate the methylating class of chemotherapeutic agents and to study the resistance to these agents mediated by base excision repair (BER). In the past, studies of methylating agent resistance have focused on two DNA repair processes, 06-alkylguanine DNA alkyltransferase and mismatch repair. However, the bulk of DNA adducts formed by methylating agents such as temozolomide are repaired by base excision repair. The applicant's preliminary data suggest that inhibition of base excision repair provides a novel approach to overcoming tumor resistance to methylating agents, especially in cells, which express high AGT and/or have defects in MMR resulting in profound methylating agent resistance. On this basis, the specific aims of this application are: 1) to determine whether human tumor cell lines with various DNA repair defects in p53, AGT and MMR express similar levels of BER proteins, 2) to optimize inhibition of BER by the AP site binding agent, methoxyamine (MX) in vitro and study its potentiation of temozolomide (TMZ) cytotoxicity, 3) to characterize the nature of the cytotoxic killing due to MX mediated interruption of BER by measurement of cell cycle arrest, apoptosis and chromosomal aberrations, and 4) to establish optimal conditions for MX enhancement of TMZ anticancer efficacy in xenograft bearing nude mice. These studies are designed to identify BER as a new target for biochemical modulation of tumor drug resistance that can become a focus of anti-cancer drug development leading to therapeutic clinical trials with these or related agents. By targeting a defined mechanism of drug resistance to a well-understood class of compounds, it should be possible to enhance the therapeutic index of methylating agents and improve anticancer responses.